Time bases, or frequency standards, are required in a large variety of electronic devices, ranging from wristwatches and other timepieces to complex telecommunication devices. Such time bases are typically formed by an oscillator including a quartz resonator and an electronic circuit for driving the resonator into oscillation. An additional division chain may be used to divide the frequency of the signal produced by the oscillator in order to obtain a lower frequency. Other parts of the circuit may serve to adjust the frequency, for example by adjusting the division ratio of the division chain. The components of the electronic circuit are advantageously integrated onto a single semiconductor substrate in CMOS technology. Other functions, not directly related to the frequency processing, may be integrated onto the same substrate.
Advantages of quartz resonators are their high quality factor Q leading to good frequency stability and low power consumption as well as their good temperature stability. A disadvantage of typical time bases using quartz resonators however resides in the fact that two components, namely the quartz resonator and the integrated electronic circuit, are required in order to provide a high-precision frequency. A discrete quartz resonator requires board space which is scarce in many cases. For instance, a standard quartz resonator for wristwatch applications requires space of the order of 2×2×6 mm3. Moreover, additional costs are caused by the assembly and connection of the two components. Yet, space and assembly costs are major issues, especially in the growing field of portable electronic devices.
A solution to the above-mentioned problems is to provide a time base comprising an integrated resonator.
More particularly, one solution consists in providing a time base comprising a resonator and an integrated circuit for driving the resonator into oscillation and for producing, in response to the oscillation, a signal having a determined frequency, the resonator being an integrated micromechanical ring resonator supported above a substrate and adapted to oscillate, according to a first oscillation mode, around an axis of rotation substantially perpendicular to the substrate, this ring resonator comprising:                a central post extending from the substrate along the axis of rotation;        a free-standing oscillating structure connected to the central post and including an outer ring coaxial with the axis of rotation and connected to the central post by means of a plurality of spring elements; and        electrode structures disposed around the outer ring and connected to the integrated electronic circuit. Advantages on this solution reside in the fact that the time base may be fully integrated on a single substrate, is suitable for mass production and is compatible with CMOS technology. In addition, such a time base is low-priced and requires only a very small surface area on a semiconductor chip.        
An advantage of this time base lies in the fact that the micromechanical ring resonator exhibits a high quality factor Q. Quality factors as high as 2×105 have been measured. For comparison, tuning-fork quartz resonators usually exhibit values between 5×104 and 1×105 after laser trimming of the fork tines. Different design features favouring a high quality factor Q are proposed.
In addition, for a given resonant frequency, the surface area required on the substrate to form the ring resonator is small in comparison with other resonators.
The electronic circuit may advantageously be integrated on the substrate together with the micromechanical ring resonator, thereby leading to a low-priced time base. A lower price is also obtained by wafer-level packaging of the resonator using wafer-bonding technology.
It must be pointed out that ring resonators having similar features are known from sensing devices, such as angular rate sensors, accelerometers or gyroscopes. For instance U.S. Pat. No. 5,450,751 to Putty et al. and U.S. Pat. No. 5,547,093 to Sparks both disclose a micromechanical ring resonator for a vibratory gyroscope comprising a plated metal ring and spring system supported above a silicon substrate. U.S. Pat. No. 5,872,313 to Zarabadi et al. discloses a variant of the above sensor which is configured to exhibit minimum sensitivity to temperature variation. U.S. Pat. No. 5,025,346 also discloses a ring resonator for use as a micro-sensor in a gyroscope or an angular rate sensor.
None of the above-cited documents however indicates or suggests using such a type of ring resonator in an oscillator circuit to act as a frequency standard or time base. Moreover, a number of design features (e.g. the shape and number of spring elements) of the ring resonators disclosed in these documents are such that they would not be suitable for horological applications where frequency stability and low power consumption are essential. For instance, the resonating structures disclosed in U.S. Pat. No. 5,025,346 exhibit a quality factor ranging from 20 to 140 which is too low for being used in a highly precise time base in horological applications, whereas quartz resonators used in horological applications exhibit quality factors of the order of 1×104 to 1×105.
Within the scope of the above solution, various design features are proposed which lead to a high quality factor Q, a high stability of the oscillation frequency against variations in the amplitude of the driving voltage, and tolerance of fabrication process variations. In fact, one of the major objectives for an application as an oscillator is a high quality factor Q. A high quality factor Q results in a stable oscillation with low phase noise and low power consumption, as is required for horological applications.
One problem of the above solution however resides in the effect of temperature on the resonant frequency of the resonator. The resonant frequency of the ring resonator is, within the temperature range of 0 to 60° C., in good approximation, a linear function of temperature. At a resonant frequency of 45 kHz, it has been observed that the thermal coefficient of the resonant frequency is of the order of −25 ppm/° C.
Two main factors determine the temperature characteristics of the ring resonator. Firstly, Young's modulus E of the material used to realize the vibrating structure decreases with increasing temperature resulting in a reduced stiffness of the spring elements and therefore a lower resonant frequency. Secondly, due to thermal expansion, the diameter of the ring will increase with increasing temperature resulting in an increased mass moment of inertia of the structure, which, in turn, also reduces the resonant frequency.
One solution to the above problem may consist in integrating a temperature measuring circuit on the substrate in order to compensate for the effect of temperature on the frequency of the signal produced by the time base. Such compensation of the resonator's temperature dependency may easily be effected since the above ring resonator has the advantage of exhibiting substantially linear temperature characteristics.
Another solution to the above problem may consist in forming a second micromechanical ring resonator on the substrate in order to allow temperature compensation.